1. Field of the Invention
The invention relates to a heating device for heating the ink in the printing head of an ink jet printer with several ink channels ending in exit openings, where the ink channels are connected to a joint ink volume space, and with individually controllable electrothermal converter elements coordinated to each ink channel, where the converter elements effect a droplet-wise ejection of ink from the exit openings.
2. Brief Description of the Background of the Invention Including Prior Art
A known principle for generation of characters on a recording substrate is based on the ejection of individual ink droplets, under the effect of a control, from the nozzles of a print head, where the print head is part of an ink printing device. By tuning and adjustment between the ejection of individual droplets and the relative motion between the recording substrate and the print head there are thereby generated characters and/or graphic patterns on the recording substrate like a grid. The operational safety and the quality of the recordings depend to a large extent on the uniformity of the ejection of the droplet. This means that the individual droplets, ejected by a control pulse have to exhibit a defined size and they have to leave the nozzle of the print head in every case at the same speed. The influence of the viscosity of the ink is very substantial for a uniform droplet ejection. The viscosity of the ink depends to a large extent on the temperature. Therefore, it is already known to maintain the temperature of the ink in an ink print head at a constant value. It is further known to furnish a heating element in the nozzle plate, according to the German Printed Patent Document Laid Open DE-OS 2,659,398 for a print head, where the individual ink channels are furnished, which ink channels end at the exit nozzles of a nozzle plate. It is further known to furnish for such print heads an induction coil in the area of the nozzle plate and to heat the nozzle plate by eddy currents and hysteresis losses as taught in the German Printed Patent Document Laid Open DE-OS 3,500,820.
More recently it has become known to achieve the ejection of individual ink droplets by generating an ink vapor bubble in the region of an electrothermal energy converter disposed in the ink channel. The ink vapor bubble defines a certain ink volume which is ejected as a droplet out of the ink channel. Such a print head can be constructed according to a thin layer technology. The temperature dependence on the viscosity of the ink is a very substantial factor for print heads of this kind. Therefore, it is further known for such print heads to improve the ejection coefficient by heating of the ink. This heating of ink can be provided by additional heating elements furnished for acting from the outside onto the ink, as taught, for example, in the German Printed Patent Documents Laid Open DE-OS 2,943,164 and DE-OS 3,545,689. Positive temperature coefficient resistors are frequently used for such heating elements. The temperature of the ink in the print head can be brought to and maintained at a certain value in connection with a control and a temperature sensor element. Frequently, a negative temperature coefficient resistor is employed as a temperature sensor element However, there result typically long heat-up times, in particular in connection with a print head with electrothermal converters. The reason for the relatively long heat-up times is based on that means for cooling have to be provided for print heads with electrothermal converters because of the heating of the ink occurring during continuous printing operation. For this purpose, the print head is usually disposed on a cooling surface, for example, on an aluminum plate. If, the ink has to be heated up after longer intervals of rest or non-use of the machine or of the printer, or upon switching on of the ink print device, then the cooling face has always to be heated at the same time. Relatively long heat-up times result by this process. In addition, the expenditure as far as the construction and production technology is concerned is not unsubstantial, since in each case additional individual elements have to be maintained ready, have to be mounted, and have to be electrically connected.
It is in fact already known from the German Printed Patent Document Laid Open DE-OS 2,943,164 either to dispose a heating coil in the interior of the ink volume space (direct heating) or to dispose a heating coil also in the ink volume space where however a coating or covering of the heating coil is to be provided (indirect heating). In the first case, in addition to the constructive expenditure, such as for example large-volume disposition, additional problems can arise in that the ink fluid reacts chemically at the heating surface which can cause deposits.
A further possible embodiment according to this reference comprises that the electrothermal converters are covered with a preheating device by the addition of a layer and include a temperature control device. This allows to react to changes in the ambient conditions.